CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

16. Community Interactions

16.2. Niche and Habitat

One of the ways scientists learn how communities work is by focusing on the activities and impacts of certain species. Each organism has a role to play and is involved in a complex set of interactions with other organisms.

The Niche Concept

The niche of an organism is its specific functional role in its community. A complete description of an organism’s niche involves a detailed understanding of the impacts an organism has on its biotic and abiotic surroundings, as well as all the factors that affect the organism. Therefore, a complete description of the niche of an organism is probably impossible. If we look hard enough, we discover something new we didn’t understand before.

For example, the niche of an earthworm includes how an earthworm is affected by abiotic factors such as the size of soil particles, soil texture, moisture, pH, and temperature. Its niche also includes biotic factors such as parasites that infect earthworms; birds, moles, and shrews that eat earthworms; dead plant material that earthworms use for food, and anglers who use worms as bait (figure 16.3). In addition, earthworms perform several important functions in their communities. They (1) transport minerals and nutrients from deeper soil layers to the surface, (2) bury seeds, (3) incorporate organic matter into the soil, and (4) create burrows, which allow air and water to penetrate the soil more easily. And this entire discussion represents only a limited sample of all the aspects of an earthworm’s niche.

FIGURE 16.3. The Niche of an Earthworm

The niche of an earthworm involves a great many factors. It includes the fact that the earthworm is a consumer of dead organic matter, a source of food for other animals, a host to parasites, and bait for an angler. Furthermore, the earthworm loosens the soil by burrowing and “plows” the soil when it deposits materials on the surface. Additionally, the pH, texture, and moisture content of the soil have an impact on the earthworm. This is but a small part of what the niche of the earthworm includes.

Some organisms have rather broad niches; others, with very specialized requirements and limited roles to play, have niches that are quite narrow. The opossum (figure 16.4a) is an animal with a very broad niche. It eats a wide variety of plant and animal foods, can adjust to a wide variety of climates, is used as food by many kinds of carnivores (including humans), and produces large numbers of offspring. By contrast, the koala of Australia (figure 16.4b) has a very narrow niche. It can live only in areas of Australia that have specific species of Eucalyptus trees, because it only eats the leaves of these trees. Furthermore, it cannot tolerate low temperatures and does not produce many offspring. As you might guess, the opossum is expanding its range, and the koala is endangered in much of its range.

FIGURE 16.4. Broad and Narrow Niches

(a) The opossum has a very broad niche. It eats a variety of foods, is able to live in a variety of habitats, and has a large reproductive capacity. It is generally extending its range in the United States. (b) The koala has a narrow niche. It feeds on the leaves of only a few species of Eucalyptus trees, is restricted to relatively warm, forested areas, and is generally endangered in much of its habitat in Australia.

Because an organism’s niche includes a complex set of interactions with its surroundings, it is often easy to overlook important roles played by some organisms. For example, Europeans introduced cattle into Australia where there had previously been no large, hoofed mammals. When this was done, no one thought about the effects of cow manure on the biotic community or the significance of dung beetles. In parts of the world where dung beetles exist, they rapidly colonize fresh dung and cause it to be broken down. Although there were dung beetles in Australia, they were adapted to feeding on the dung of native marsupials and did not colonize cow dung. Therefore, in areas of Australia where cattle were raised, a significant amount of land became covered with accumulated cow dung. This reduced the area where grass could grow and reduced productivity. The problem was eventually solved by the importation of several species of dung beetles from Africa and Europe, where large, hoofed mammals are common. The dung beetles made use of what the cattle did not digest, returning it to a form that plants could recycle more easily into plant biomass.

The Habitat Concept

The habitat of an organism is the kind of place or community in which it lives. Each organism has particular requirements for life and lives where the environment provides what it needs. Habitats are usually described in terms of a conspicuous or particularly significant feature. For example, the habitat of a prairie dog is usually described as grassland and the habitat of a tuna is described as the open ocean. The habitat of a fiddler crab is sandy ocean shores and the habitat of various kinds of cacti is the desert. It is possible to describe the habitat of the bacterium Escherichia coli as the gut of humans and other mammals and the habitat of a fungus as a rotting log. Organisms that have very specific places in which they live simply have more restricted habitats.